Cooldown strategies for ship-borne cryogenic storage tanks during the ballast voyage

The transition toward decarbonisation has driven recent interest in liquid hydrogen (LH2) export. Like liquefied natural gas (LNG), LH2 must be stored at cryogenic temperatures. After unloading, near-empty tanks may warm up during the return (ballast) voyage. To prevent excessive boil-off gas (BOG) generation during loading, tank walls must be pre-cooled. For liquefied natural gas (LNG), this is achieved by reserving a quantity of liquid (heel) which is sprayed onto the walls. Since carrying heel reduces cargo delivered, it is important to understand how cooldown strategies may affect later BOG generation. This study investigated and compared strategies for LNG and LH2 carriers using analytical models for heat and mass transfer. Tank insulation is discretised as a 2-D thermal network, while vapour and liquid are treated as lumped masses. For both tanks, selecting a strategy involved balancing roundtrip losses and laden voyage losses. Allowing warm-up during the ballast voyage reduced losses by ∼50 %, compared to keeping the tank cold, but resulted in excess BOG generation up to 5.3 % for LH2 and 17.2 % for LNG during loading and the laden voyage. Optimal strategies may ultimately depend on other factors such as equipment sizing and BOG consumption profiles for ship power.